When nonmetal atoms gain electrons to become ions, they typically form negative ions. This is because nonmetal atoms tend to have higher electronegativity, meaning they have a greater attraction for electrons.
When a nonmetal atom gains one or more electrons, it exceeds the number of protons in its nucleus, resulting in a negative charge. This happens because electrons are negatively charged particles, and adding more of them to an atom creates an overall negative charge.
By gaining electrons, nonmetal atoms achieve a stable electron configuration, typically by filling their valence electron shell. A full valence shell is the most stable arrangement of electrons, similar to the noble gases. Therefore, nonmetals gain electrons to reach a configuration that resembles the nearest noble gas, which typically has a complete valence shell.
For example, oxygen (O) has six valence electrons, and it requires two additional electrons to achieve a stable configuration like neon (Ne), which has a complete valence shell. By gaining two electrons, oxygen forms the oxide ion (O^2-), which has a negative charge due to the excess electrons.
In summary, nonmetal atoms become negative ions when they gain electrons because it allows them to achieve a more stable electron configuration and resemble the electron arrangement of noble gases.